Transflective liquid crystal display using separate transmissive and reflective liquid crystal cells and materials with single cell gap

ABSTRACT

A transflective liquid crystal display (TLCD) using separate transmissive (T) and reflective (R) cells in which two liquid crystal materials with different birefringence changes are used. The birefringence change of the R region is half of the birefringence change of the T region. In this case, a single cell gap is possible and identical transmittance and reflectance for R and T is obtained. It is applicable to various reflective LC modes, and the fabrication methods are simple.

This application is a divisional Application of U.S. patent applicationSer. No. 10/915,070 filed on Aug. 10, 2004 now U.S. Pat. No. 7,359,015.

This invention relates to transflective liquid crystal displays, and inparticular to structures and fabrication methods for transflectiveliquid crystal displays (LCDs) having a single cell gap that useseparate transmissive (T) and reflective (R) cells containing liquidcrystal materials that differ only in the birefringence of the R region,which is half that of the T region.

BACKGROUND AND PRIOR ART

The transmissive liquid crystal display (LCD) exhibits a high contrastratio and good color saturation. However, its power consumption is highdue to the need of a backlight. At bright ambient, the display could bewashed out completely. On the other hand, a reflective LCD uses ambientlight for reading displayed images. Since it does not require abacklight, its power consumption is reduced significantly. However, itscontrast ratio is lower and color saturation much inferior to those ofthe transmission type. At dark ambient, a reflective LCD loses itsvisibility. Transflective LCDs use a combination of transmissive andreflective modes to provide improvements in image display and powerconsumption.

Two types of transflective LCDs have been developed: single cell gap(FIG. 1) and double cell gap (FIG. 2).

A single cell transflective LCD is disclosed in U.S. Pat. Nos. 6,281,952B1 to Okamoto et al.; 6,295,109 B1 to Kubo et al.; 6,330,047 B1 to Kuboet al., commonly assigned to Sharp Kabushiki Kaisha, which use asplit-pixel approach, i.e. each pixel is split into reflective (R) andtransmissive (T) sub-pixels. Usually, the R and T area ratio is 4:1, infavor of the transmissive display. The transmissive display is used fordark ambient only in order to conserve power.

In the conventional single cell gap approach shown in FIG. 1, the cellgap (d) 100 for R and T modes is the same. The cell gap is optimized forR-mode. As a result, the light transmittance for the T mode is lowerthan 50% because the light only passes through the LC layer once.

In the conventional double cell gap approach 200 shown in FIG. 2, thetransflective LCD has separate transmission and reflection pixels inorder to compensate the unmatched liquid crystal retardation. The cellgap is d and 2d for the R and T pixels, respectively. In this approach,both R and T have high light efficiency. However, the T mode has fourtimes slower response time than that of the R mode. Moreover thisapproach has a complicated structure and fabrication process. Glassetching and indium-tin-oxide (ITO) electrode coating on the transmissionregion are needed. The cell gap accuracy and uniformity can be poordepending critically on how accurate and uniform the extra thick organiclayer is formed. Poor cell gap accuracy and uniformity result indeteriorated LCD performances, such as variations in brightness andcolor.

U.S. Pat. No. 6,020,941 to Yao-Dong Ma employs switchable liquid crystalmaterials of two polarities in separate channels, a wall located in aninterstice between the separate channels defines a first and a secondset of independent cells in the LCD. A first controllable liquid crystal(CLC) material is located in the plurality of independent cells, thefirst CLC material selectively exhibits an “on” state and an “off” stateand has a first polarity when in the “on” state; and a second CLCmaterial located in the plurality of independent cells, the second CLCmaterial selectively exhibits an “on” state and an “off” state and has asecond polarity when in the “on” state.

Another cell wall structure is disclosed in U.S. Pat. No. 4,720,173 toOkada et al. and is used to improve the alignment or orientation of theliquid crystal molecules. There remains a need to improve the quality ofliquid crystal displays and to provide them at lower costs.

SUMMARY OF THE INVENTION

A primary objective of the invention is to provide a new transflectiveliquid crystal display having single cell gap.

A secondary objective of the invention is to provide a new transflectiveliquid crystal display using separate T and R cells.

A third objective of the invention is to provide a new transflectiveliquid crystal display, in which T and R cells are filled with twoliquid crystal materials that differ only in the birefringence of the Rregion to half of the T region.

A fourth objective of the invention is to provide a new transflectiveliquid crystal display with improved LCD quality with improved cell gapcontrol since only single cell gap is required.

A fifth objective of the invention is to provide a new transflectiveliquid crystal display with a simpler structure and fabrication processcompared with the double cell-gap approach. No spacer is needed sincethe wall which separates the R and T regions also act as the LCD spacer.

A sixth objective of the invention is to provide a new transflectiveliquid crystal display without a response time difference because ofsingle cell-gap.

A seventh objective of the invention is to provide a new transflectiveliquid crystal display at a lower cost since no major extra component isrequired.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodimentwhich is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a transflective LCD using a single cell gap (Prior Art).

FIG. 2 shows a transflective LCD using a double cell gap (Prior Art).

FIG. 3 shows the operating principle of a transflective LCD of thepresent invention, using a single cell gap with different birefringencefor T and R regions.

FIG. 4 shows the structure of a transflective LCD of the presentinvention, using a single cell gap with different birefringence for Tand R regions.

FIG. 5 shows the serpentine wall structure that defines the T and Rindependent cells.

FIG. 6 shows the reflection and transmission vs. applied voltage of thetransflective LCD using a vertically aligned LC cell as an example.

FIG. 7 shows normally white transflective LCD using a homogeneous cellof a mixed-mode twisted nematic (MTN) cell.

FIG. 7 a shows the normally white transflective LCD of FIG. 7 in thebright state at voltage V=0.

FIG. 7 b shows the normally white transflective LCD of FIG. 7 in thedark state at V=ON.

FIG. 8 shows a normally black transflective LCD employing a verticallyaligned LC cell.

FIG. 8 a is a normally black transflective LCD employing a verticallyaligned LC cell in the dark state at V=0.

FIG. 8 b is a normally black transflective LCD employing a verticallyaligned LC cell in the bright state at V=ON.

FIG. 9 shows the structure of a transflective LCD of the presentinvention with a single cell gap with different birefringence for T andR regions and having a single λ/4 film and two polarizers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangements shown sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

In the present invention, when referring to the “transmissive ortransmission” and “reflective or reflection” modes, the terms aresometimes abbreviated with T and R, respectively.

Two liquid crystal materials are used to fill two independent regions.Except for birefringence, the liquid crystal materials have all otherproperties that are the same. The birefringence satisfies the followingequation:Δn_(T)˜2Δn_(R)where Δn_(T) is the birefringence of liquid crystal in T region andΔn_(R) is the birefringence of liquid crystal in R region. The presentinvention is different from the above prior arts. Because ofbirefringence, the light passing through a liquid crystal layer wouldexperience an optical path difference which is defined as δ_(T)=dΔn_(T);where d is the LC cell gap. For the reflective pixels, the light passesthe LC layer twice so that the incurred optical path difference isdoubled, as shown by the following equation:δ_(R)=(Δn _(R) ×d)×2Therefore, the optical path difference (or phase retardation) for the Rand T pixels are equal:δ_(R)=δ_(T)Thus, the transmissive and reflective pixels will have the same lightefficiency. That means if the T pixels exhibit 100% transmittance, thenthe R pixels will have 100% reflectance. Their gray scale performancewill be very similar. This is particularly important when both T and Rpixels are operating simultaneously.

In order to make T and R operate with the same characteristics, thereare requirements for liquid crystal materials for this invention. The LCbirefringence meets Δn_(T)=2Δn_(R). For example Δn_(T)=0.14,Δn_(R)=0.07. Other properties such as dielectric constants, splayelastic constants, and rotational viscosity should be maintained roughlythe same to guarantee T and R regions have the same threshold voltage,response time and the like. The two-bottle liquid crystal mixtures(e.g., MLC-9200-000 and MLC-9200-100) developed by MERCK in Darmstadt,Germany have very similar physical properties, such as, phase transitiontemperatures, dielectric constants, and viscosity, except for differentbirefringence. Although the birefringence is not exactly different by afactor of two, further improvement can be made based on thespecifications. Other research laboratories, such as the Photonics andCommunications Laboratory at College of Optics and Photonics, Universityof Central Florida, can also formulate acceptable liquid crystalmixtures for the present invention.

Referring now to the transflective LCD 30 shown in FIG. 3, the cell wall32 that defines the independent T and R cells 34, 36 is a serpentinewall structure 60 as shown in FIGS. 4 and 5. The cell wall 32 separatesthe T cell 34 from the R cell 36 and also acts as a spacer. Ambientlight passes through a top linear polarizer into the reflective cell 36of the liquid crystal and is reflected back. A backlight source faces abottom linear polarizer layer and passes light through a transparentarea located below the transmissive cell 34.

FIG. 4 provides further detail on the structure and fabrication processused to provide the novel transflective LCD 40 of the present invention.A liquid crystal material is sandwiched between a first transparentsubstrate 42 coated with an indium-tin-oxide ITO electrode layer 44 andan alignment layer 46 and a second transparent substrate 52 also coatedwith an ITO electrode layer 54 and an alignment layer 56. A wall 32 witha serpentine structure 60 divides the liquid crystal materials intoindependent cells T 34 and R 36. The wall structure 60 is held in placewith an adhesive 48 which seals the wall 32 in place and also seals theperiphery of the T and R cells.

FIG. 5 shows greater detail of the serpentine wall structure 60 as wellas the fluid fill ports 62, 64 for filling the T and R cells, onopposing edges of the substrate. The set of independent T cells can befilled with a T liquid crystal material using a conventionalvacuum-filling method, and then sealed using, for example, an epoxy. Theset of independent R cells can then be filled with R liquid crystalmaterial, again using a conventional vacuum-filling method, and thensealed.

The cell wall is designed just under the black matrix, so the apertureratio for the present invention is the same as a conventional LCD. Thewall structure may be of a type disclosed in U.S. Pat. Nos. 6,020,941and 4,720,173, and the cell wall structures are incorporated herein byreference. The present invention is not restricted to a particular cellwall structure.

Referring to the fabrication of a cell wall, similar to U.S. Pat. No.4,720,173, a polyimide film is formed in a thickness of 1000 Å on one ofthe transparent substrates or base plates. A polyimide film is formed onanother transparent substrate or base plate in a cell gap thickness ofapproximately 5 micrometers (μm) for the present invention, which isthen photoetched to leave a spacer stripe of approximately 10-20 μmwidth, equal to the width of the black matrix.

After the wall forming, the alignment process is the same as for aconventional LCD. For a transflective LCD, the pixel is approximately240 μm and sub-pixel is approximately 80 μm, and even the wall isapproximately 20 μm, the pixel width is much larger than the cell gap ofapproximately 5 μm. Therefore the alignment, rubbing or tilt vapordeposition is not affected by the wall.

FIG. 6 shows the modeling results for the reflection and transmissionvs. applied voltage of the transflective LCD of the present invention.It is obvious that both R and T reach 100% at V=5 V_(rms).

Several outstanding features of the novel transflective LCD include butare not limited to, the features summarized below.

First, the transflective LCD of the present invention provides identicaltransmittance and reflectance for R and T cells, since T and R have thesame retardation change, both have high, approximately 100%, lightmodulation efficiency, as shown in FIG. 6.

Second, in the present invention a single cell gap LCD is used, as shownin FIGS. 3 and 4. Because of the same cell gap, the R and T pixels havethe same response time. The structure and fabrication process aresimple. No spacer is needed since the wall which separates the R and Tregions also act as the LCD spacer.

Third, the transflective LCD of the present invention is versatile.Several reflective LC modes can be considered. Below is an example ofthree different liquid crystal modes that can be embodied in the presentinvention. For examples, the mixed-mode twisted nematic (MTN) cell withvarious twist angles (90°, 80°, or 75°) offer normally white operation.The 90°-MTN cell exhibits a high contrast, but its reflectance islimited to ˜88%. On the other hand, the 80° and 75° MTN cells havehigher reflectance (˜100%), but their contrast ratio is lower. In theMTN embodiment, a single λ/4 film and two polarizers are needed asdisclosed by Wu and Yang, “Reflective Liquid Crystal Displays”(Wiley-SID, 2001), Ch. 4.

Another variation is the film-compensated homogeneous LC cell with λ/4phase retardation, for example as shown in FIG. 9. Similar to theabovementioned examples, this embodiment includes a single λ/4 film andtwo polarizers as discussed by Wu and Yang, “Reflective Liquid CrystalDisplays” (Wiley-SID, 2001), Ch. 3.

A further example of the versatility of the present invention, thevertical alignment LC cell with λ/4 phase retardation can also beembodied. In this embodiment, two λ/4 films and two polarizers areneeded.

FIG. 7 illustrates the operating principles of the transflective LCD ofthe present invention employing a homogeneous LC cell as an example. Themixed-mode twisted nematic (MTN) cell should have a higher contrastratio and lower operating voltage. In the V=0 state in FIG. 7 a, thepolarization of the back light and ambient light are shown in eachstage. Here, the half circle symbols represent the circularpolarization. With the arrow facing right (or left), they are right (orleft) handed circular polarization. FIG. 7 b is for V=ON state. Thetransmission and reflection of light in the separated T and R pixelsprovide a high quality display of images in any ambient lightconditions. The major advantage of the MTN and film-compensatedhomogeneous cells is their simple fabrication process and low cost.

Similarly, FIG. 8 depicts a normally black mode using a verticalalignment LC cell. In the voltage-off state shown in FIG. 8 a, the LCdirectors are perpendicular to the glass substrates. The effective phaseretardation δ=2πdΔn/λ is equal to zero. As a result, both ambient andback light are blocked by the crossed polarizers. In the voltage-ONstate, shown in FIG. 8 b, the transmissive part of the cell remainsunaffected because of no electrode. However, the reflective sub-pixel isactivated. The effective phase retardation is δ=π/4 so that the lightleaks through the crossed polarizer. The transmission and reflection oflight in this transflective LCD gives very sharp, clear images. Themajor advantage of the vertical alignment is its high contrast ratio.For the T pixels, the contrast ratio could exceed 500:1. For thereflective pixels, the contrast ratio exceeds 50:1. The lower contrastratio for the reflective LCD is because of the surface reflections fromthe optical components and substrates.

Another outstanding feature of the present invention is the simplicityof the fabrication process. In view of the above descriptions, the novelinvention structure is very compatible with present manufacturingtechniques. The only extra step in the current fabrication process is tobuild a wall on the first and second substrate, but no spacer is neededsince the wall also acts as the LCD spacer. A high performance and lowcost transflective LCD is provided.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A method of improving a transflective liquid crystal display (TLCD)comprising the steps of: providing a liquid crystal area structurehaving a transmissive portion and a reflective portion in the liquidcrystal area of the transflective LCD comprising the steps of:fabricating a serpentine wall on a transparent substrate; sealing theserpentine wall to the substrate to form independent transmissive andreflective regions having an equal cell gap; filling the transmissiveportion with a first liquid crystal material; filling the reflectiveportion with a second liquid crystal material, a birefringence of thesecond liquid crystal material being one-half a birefringence of thefirst liquid crystal material in the transmissive portion; and passingbacklight and ambient light through the TLCD to achieve identicaltransmittance and reflectance displaying high quality images.
 2. TheTLCD of claim 1, further including: a single quarter-wave (λ/4) film anda top polarizer layer and a bottom polarizer layer.
 3. The TLCD of claim1, further including; a quarter-wave (λ/4) film adjacent to a toppolarizer layer and a quarter-wave (λ/4) film adjacent to a bottompolarizer layer.
 4. The method of claim 1, wherein the first and secondliquid crystal materials are a mixed-mode twisted nematic (MIN) cell. 5.The method of claim 1, wherein the first and second liquid crystalmaterials are a homogeneous liquid crystal with quarter wave phase (λ/4)retardation.
 6. The method of claim 1, wherein the first and secondliquid crystal materials are a vertically aligned liquid crystal withquarter wave (λ/4) phase retardation.